N-vinyl lactam polymers are represented by, for example, polyvinyl pyrrolidone and used in various fields as safe functional polymers. They are used, for example, in cosmetics, medical and agrochemical intermediates, food additives, photosensitive electronic materials, and tackifiers, and also in specialized industrial applications (e.g., production of hollow fiber membranes). In particular, polyvinyl pyrrolidone having a low molecular weight is usable in these applications.
Polyvinyl pyrrolidone having a low molecular weight is generally produced by polymerization of N-vinyl-2-pyrrolidone in an aqueous medium in the presence of a metal catalyst with use of hydrogen peroxide as a polymerization initiator (see Patent Literatures 1, 2, and 3). For example, a disclosed method for producing polyvinyl pyrrolidone having a comparatively low molecular weight safely at a low temperature in a short time includes the step of polymerizing a monomer component containing vinylpyrrolidone in the presence of hydrogen peroxide, a metal catalyst, and at least one of ammonia and an amine compound (see Patent Literature 3).
The production method in which hydrogen peroxide, a metal catalyst, and at least one of ammonia and an amine compound are used, however, has a problem that the produced polyvinyl pyrrolidone is colored in brown or yellow in polymerization or storage. Such colored polyvinyl pyrrolidone is not acceptable depending on the application. The cause of coloring is presumably oxidation of a product material by hydrogen peroxide, oxidation of an amine compound, and promotion of the oxidation of the product material by ammonia in a combination system that includes hydrogen peroxide, a metal catalyst, and at least one of ammonia and an amine compound.
As a method for suppressing coloring of polyvinyl pyrrolidone during polymerization and storage, disclosed is a method having a step of treatment with a cation-exchange resin during and/or after polymerization of a monomer component containing vinylpyrrolidone in the presence of hydrogen peroxide, a metal catalyst, and at least one of ammonia and an amine compound for production of polyvinyl pyrrolidone (see Patent Literature 4). The method is industrially usable as a method for producing polyvinyl pyrrolidone that has a comparatively low molecular weight and is less likely to be colored during polymerization and storage.
A dye transfer inhibitor containing a polymer that contains 90% by mass or more of a vinyl lactam unit and has a K value of 28 or less is disclosed, the dye transfer inhibitor containing a chain transfer agent that has an acid group (see Patent Literature 5). Patent Literature 5 teaches that a chain transfer agent having an acid group is bonded to end of a polymer including a vinyl lactam unit to have an enhanced effect on a dye and exhibit its performance as a dye transfer agent especially in the presence of an anionic surfactant. Patent Literature 5 also discloses poly-N-vinylpyrrolidone that is produced with use of a mixture of 4,4′-azobis-4-cyanovaleric acid (0.94 parts) and triethanolamine (0.98 parts) as a polymerization initiator and ammonium hypophosphite as an chain transfer agent, and has a K value of 17.3.